The invention relates to electrical oscillations, and in particular to crystal oscillators where the frequency may be slightly perturbed by factors such as the past thermal history, acceleration, or time such as aging.
Temperature compensated crystal oscillators (TCXO) and oven controlled crystal oscillators (OCXO) are well known in the art, see for example: Crystal Oscillator Design and Temperature Compensation, by Marvin E. Frerking, Van Nostrand Reinhold, 1978. In a Digitally Temperature Compensated Crystal Oscillator, the temperature is measured by a temperature sensor and converted to digital form by an A/D converter. A thermistor of other resistance that is variable with temperature may be used as the sensor. Other means such as temperature sensitive IC""s may be used as the sensor. The memory of the microcomputer is programmed to store the temperature characteristics of the reference crystal, usually at specific fixed temperatures. The temperature in digital form is used to address the memory. The closest stored values are used for interpolation to find the correction required at the exact ambient temperature being measured. The compensation value is then converted to an analog voltage by a D/A converter and applied to a voltage variable element in the oscillator to correct its frequency. Using this technique, however, residual effects tend to limit the ultimate accuracy that can be achieved. For example in a temperature compensated crystal oscillator, the accuracy of the compensation is limited by frequency hysteresis resulting from the past thermal history of the crystal, e.g. the frequency at a given temperature is slightly different depending on whether the crystal has been cold or hot last, how long ago, and the extent of the temperature excursion.
In the case of oven controlled crystal oscillators, the frequency may be slightly different after the unit is turned off and on again allowing the crystal to see a significantly different temperature while the unit is turned off. It is also well known that the frequency of a crystal resonator gradually changes with time (aging).
Another residual effect in crystal oscillators is due to orientation in the earth""s gravitational field which may limit the accuracy of a precision oscillator.
Heretofore, there has been no effective means of internally sensing these residual effects and compensating for them, particularly the effects of temperature hysteresis. In some applications it has been possible to predict the crystal aging rate by observing it compared it to an external standard for a period of time, and then by assuming that the aging rate remains constant, correct for it after the external standard is removed for a short period of time. This is only effective if the aging rate is fairly constant and the crystal temperature does not change. Some experimental compensation for aging has been done by sensing the frequency difference between the desired mode and a closely spaced anharmonic mode by modulating the oscillator to sense the anharmonic resonance frequency. There has been no effective way, however, to predict and compensate for hysteresis effects.
In the subject invention, as will be described subsequently, it will be noted that these hysteresis effects in modes significantly separated in frequency or vibrating in a different way are not necessarily affected in exactly the same way, therefore, it is possible to use these differential effects as a hysteresis sensor, or aging sensor and compensate for them to some extent thus improving the overall accuracy.
The present invention consists of the use of multiple modes of oscillation in a crystal oscillator either simultaneously or sequentially that are affected differently by hysteresis, aging, or acceleration so that by measuring the change in differential frequencies, a proportional correction can be made to the main mode and improve the frequency accuracy. When sequential frequency modes are used, a second oscillator against which frequency measurements are made is required. The technique can also be applied with multiple oscillators rather than multiple modes of the same resonator when the oscillators are exposed to the same environment.
The invention is applicable to temperature compensated crystal oscillators and to oven controlled crystal oscillators.